Lithographic processes involve establishing image (printing) and non-image (non-printing) areas on a substrate, substantially on a common plane. When such processes are used in printing industries, non-image areas are generally hydrophilic and image areas are generally oleophillic. Consequently, oil based inks are repelled from the non-image areas after water has been applied to the substrate.
Image and non-image areas can be created by processes which include a step of exposing a layer of image material on the surface of the substrate to radiation. The exposure to radiation creates solubility differences in the image material corresponding to image and non-image areas. During development, the more soluble areas are removed, leaving a pattern on the substrate corresponding to the image.
Preparation of the substrate for receiving a layer of the image material must ensure that the image material bonds to the substrate. However, it must allow release of the soluble image material during development.
One of the most common substrates used in lithographic printing comprises an aluminum base layer which is treated to make it suitable for use. In general, the aluminum layer comprises high quality aluminium, for example 1050 alloy which is at least 99.5% pure. For preparation of a substrate, the aluminum is roughened, for example by electrograining, anodized and then conditioned by chemical means, for example by treatment with water, a solution of phosphate or silicate salt, or a polycarboxylic acid.
Lithographic printing plates which utilize electrograined and/or anodized and/or chemically conditioned aluminum are described in, for example, UK Patent Application No. 1 439 127, U.S. Pat. Nos. 3,181,461, 3,963,594, 4,052,275, 4,072,589, 4,131,518, European Patent Application No. 0 110 417 and Japanese Publication No. 20/3956.
One problem with the known processes is that they consume a significant amount of electrical energy in the electrograining and anodizing steps. Furthermore, these steps produce waste chemicals which must be disposed of. Additionally, the processes can generally only be run at relatively low speed.
Numerous solutions have been proposed for the above described problems; however, few such proposals have been used commercially.
For example, PCT Publication No. WO91/12140 discloses a lithographic plate of aluminum metal which carries an oxide layer derived from a zirconia sol.
U.S. Pat. No. 4,457,971 discloses a lithographic printing plate comprising an aluminum or aluminized substrate bearing a ceramic layer comprising non-metallic inorganic particles and a water resistant phase or phases of a dehydration product of at least one monobasic phosphate.
U.S. Pat. No. 4,420,549 discloses a lithographic printing plate comprising an aluminum or aluminized substrate bearing a ceramic coating comprising a polymeric form of aluminum phosphate or mixtures of aluminum phosphates wherein the coating is substantially free of particulate matter.
U.S. Pat. No. 4,542,089 discloses a process for preparing a photosensitive substrate comprising providing a hydrophilic ceramic on an aluminum substrate or aluminized surface of a substrate by applying a slurry of at least one monobasic phosphate and inorganic non-metallic particles on at least one surface of the aluminum or aluminized substrate and firing the slurry at a temperature of at least 230.degree. for a time sufficiently long to ensure substantially complete dehydration of the ceramic layer to form a hydrophilic ceramic coating.
Italian Patent Application No. MI94 A000448 describes lithographic plates prepared by applying a colloidal mixture comprising fluorosilicate, silica, polyvinylidene fluoride and titanium dioxide to an aluminum support. Polymerization of the fluorosilicate is carried out at 225.degree.-300.degree. C. for 50-180 seconds.
One problem associated with the above described processes results from the relatively high temperature required to cure and/or polymerize the coating on the aluminum. High temperatures are found to anneal the aluminum support and reduce its tensile strength. Additionally, high temperatures may deform the plate and cause it to have a wavy structure. Both of these effects can be problematical when the plates are run on a printing press.
Another solution to the problem of electrograining and/or annealing is described in PCT Patent Application No. GB93/01910. The document discloses making a lithographic printing plate by plasma spraying Al.sub.2 O.sub.3 powder onto aluminum alloy sheet.
As an alternative to aluminum, plastic materials, for example polyesters, may be used as supports. Again, there are numerous disclosures of surface coatings for such materials.
For example, U.S. Pat. No. 4,330,605 discloses a photolithographic receptor sheet capable of being imaged by a silver salt diffusion transfer process which comprises coating a polyethylene terephthalate film with a mixture of colloidal silica and dry silica powder.
EP 0 619 524, EP 0 619 525 and EP 0 620 502 also disclose various coatings for polyethylene terephthalate film.